1. Field of Use
This invention relates to an improved process for producing TiO.sub.2 from ilmenite-type ores wherein undesirable iron impurities are removed from an acid solution of the digested ore by first converting the iron impurities to the ferric form and then by conducting a liquid-liquid extraction of the ferric iron impurities using an organophosphoric acid, or salt thereof, as the extracting agent.
2. Brief Description of the Prior Art
Titanium dioxide is a well known material which is useful as a pigment for a wide variety of applications such as interior and exterior paints and automotive finishes. A main source for the production of TiO.sub.2 is from ilmenite-type ores which are generally of the formula TiO.sub.2.FeO. A prior art process for treatment of the ore is described in U.S. Pat. No. 2,042,435 (1936) which involves removal of iron impurities from the digestion step, prior to isolation of TiO.sub.2, because of the adverse affect of the impurities on the final color and pigmentary properties of formed TiO.sub.2. The ore may be treated with aqueous hydrogen fluoride, aqueous ammonium fluoride or dry ammonium fluoride, in the initial digestion step, and subsequently, ammonium fluoride solution is added to substantially precipitate the iron impurities as insoluble iron ammonium fluoride salts. However, this step necessitates the use of large amounts of aqueous ammonium fluoride solutions which adds to the total amount of liquid aqueous solutions which must be handled and recycled in the process. Alternate processes for separating out the iron impurities which do not involve large aqueous volumes would be highly desirable.
The use of organophosphoric acids in extracting metal values from aqueous solutions has been described in the art.
The reference, J. Phys. Chem., 64, 89-94 (1960), describes the use of di-(2-ethylhexyl)orthophosphoric acid in n-octane solution for extracting iron (III) values from aqueous acidic perchlorate solutions.
The reference, Solvent Extr. Proc. Int. Solvent Extr. Conf. 1971, 1, 238-48, describes the use of a solution of di-(2-ethylhexyl)orthophosphoric acid in kerosene for extraction of iron (III) values from aqueous acidic hydrogen chloride and sulfuric acid solutions.
British Pat. No. 970,885 (1964) describes the use of mono- or dialkyl substituted orthophosphoric acids as extractants for leaching out titanium values, and the use of trialkyl phosphates for extracting iron impurities, from sulfuric acid digestion solutions of ilmenite-type ore. Specifically disclosed is the use of di-(2-ethylhexyl)orthophosphoric acid, dissolved in kerosene, for extracting out titanium values along with a small portion of iron impurities from an aqueous sulfuric solution, leaving substantially all of the iron impurities in the aqueous layer.
However, no specific mention is made in the above references as to the possible utility of the organic extractant in extracting out metal values, particularly ferric iron impurities, from aqueous acidic fluoride solutions containing dissolved titanium values.
We have unexpectedly found that iron impurities can be substantially extracted out of aqueous acidic fluoride solutions, containing titanium values by first converting the iron impurities to the ferric form and then extracting same by the use of a mono- or dialkyl, or mono- or di(alkylphenyl)orthophosphoric acid, or alkali metal or ammonium salt thereof, wherein the alkyl radical is linear or branched and contains 4 to 18 carbon atoms. Extraction yields of over about 99 numerical percent of the iron impurities present can be achieved. A preferred extractant in the process is di-(2-ethylhexyl)orthophosphoric acid.